Field devices as described in the following are generally used in a manufacturing process to monitor the process and to actuate process variables. Typically, actuators are placed in the manufacturing field to drive different process control elements, such as valves or sensors. Further, transmitters are installed in the manufacturing field to monitor process variables, such as fluid pressure, fluid temperature or fluid flow.
Actuators and transmitters are coupled to a control bus to receive process information and transmit the process information to a centralized system controller that monitors the overall operation of the manufacturing process. This control bus may be implemented as a two wire current loop carrying current that provides power supply for operation of a field device.
In such control systems, communication is typically executed through a fieldbus standard, which is a digital communication standard that permits transmitters to be coupled to only a single control bus to transmit sensed process variables to the central controller. Examples of communication standards include ISA 50.02-1992 Section 11, HART®, Foundation Field Bus, Profibus Pa., and FoxCom. HART® overlays digital communication on a 4 to 20 mA process variable signal.
An important aspect with respect to control systems of the type outlined above is intrinsic safety. When a field device is located in a hazardous area without explosion proof equipment, the electronics in the field device should be intrinsically safe, which means that the electronics must be designed so that no sparks and no heat are generated thereby even when one or more electronic component failures occur at the same time.
Usually intrinsic safety is achieved by employing additional protective elements to protect the electronics under a failure condition. Design specifications and certifications for the protective elements vary with the specific type of application. For example, they may vary with the type of explosive gas used within a manufacturing process.
FIG. 1 shows a schematic diagram of a manufacturing process control system. As shown in FIG. 1, the peripheral part of the control system may comprise a first intrinsically safe fieldbus segment 10 and a second bus segment using, e.g., the RS485 standard for data communication. The intrinsically safe fieldbus segment 10 and the RS485 bus segment 12 are coupled through a bus coupler 14. Further, the side of the intrinsically safe fieldbus segment 10 not being attached to the bus coupler 14 is connected to a terminating circuit 16 that helps to avoid reflections on the intrinsically safe fieldbus segment 10.
As also shown in FIG. 1, to each bus segment 10, 12 there is connected at least one field device 18, 20 and 22. Each field device is either an actuator, a transmitter or another I/O device receiving and/or transmitting information.
The field devices attached to the intrinsically safe fieldbus segment 10 may be powered through an electric current received from the intrinsically safe fieldbus segment 10 leading to a voltage drop across the field devices 20, 22. Typically, the intrinsically safe fieldbus segment 10 will be operated under a fieldbus protocol or any other appropriate protocol allowing to exchange digital information.
As shown in FIG. 1, the field devices 20, 22 coupled to the intrinsically safe fieldbus segment 10 exchange information through modification of the current flowing into each single field device 20, 22. For digital communication, a basic value of the current of the intrinsically safe fieldbus segment 10 is modulated to be increased or decreased by a predetermined offset value, i.e. 9 mA for the fieldbus standard. This modulation of the current flowing into either the field device 20 or the field device 22 leads to a modification of a voltage UB on the intrinsically safe fieldbus segment 10 thus achieving digital communication.
FIG. 2 shows a more detailed schematic circuit diagram of a field device shown in FIG. 1. As shown in FIG. 2, the intrinsically safe fieldbus segment 10 may be summarized into an equivalent circuit diagram with an ideal voltage source 24 and a resistor 26 to model AC voltage impedance and to fulfill intrinsic safety requirements for spark protection, current limitation and power limitation in a hazardous area. As also shown in FIG. 2, each field device is connected to the intrinsically safe fieldbus segment with two lines 28, 30 being also connected to a discharge protection unit 32. At the output of the discharge protection unit 32 there is provided a modulating unit 34 which allows modulation of the operating current flowing into the field device.
The modulating unit 34 is connected in series to a power converter unit 36 that is adapted to map the operating current flowing over the modulating unit 34 into a suitable power supply signal for a control unit 38 connected to the output of the power conversion unit 36. The control unit 38 is connected to an actuator and/or sensor unit 40 for the control thereof.
Operatively, the controller unit 38 controls the operating current modulating unit 34 to achieve a modulation of the operating current and therefore exchange information between the intrinsically safe fieldbus segment 10 and the field device. Further, the control unit 38 has control over the further elements in the field device.
Operatively, each field device 20, 22 connected to the intrinsically safe fieldbus segment 10 receives an operating current from the intrinsically safe fieldbus segment 10. During transfer of information from the field device to the intrinsically safe fieldbus segment 10, the current value for the operating current is determined by the modulating unit 34 under control of the control unit 38. Further, to receive information at the field device, the controller unit 40 maintains the resistance of the modulating unit 34 at a constant value. When a different field device triggers a change of the voltage on the intrinsically safe fieldbus segment 10, the remaining field device(s) connected to this intrinsically safe fieldbus segment 10 may detect this change of a voltage through the connection lines 28, 30 for further processing thereof in the control unit 38. This digital communication mechanism is used to provide the controller unit 40 in each field device both with control information for activation of actuators and/or sensors during manufacturing process control and surveillance of the field device itself.
It becomes clear that explosion protection in a hazardous area and shortage of energy supply are currently the major constraints for the operation of field devices. Therefore, different approaches to ignition protection in hazardous areas exist, e.g., an explosion intrinsically safe fieldbus, passive achievement of intrinsically safety through related design of electronics to avoid overheating and increased currents/voltages, or active implementation of intrinsic safety using active electronic devices such as electronic limiters. For reasons of explosion protection, if the electronics of a field device are not intrinsic safe, encapsulation into mechanically stable housings and sealed conduits and pipes for electric cables are required to achieve explosion protection, independent from the electronic design. To support both protection systems with one type of device, intrinsicly safe electronic and explosion proof mechanical design must be combined in one field device.
In conclusion, the exchange of information and the access to sensors in the field device is severely limited both from a mechanical but also from an electrical point of view and only limited transfer rates are achievable.
In other words, higher transfer rates in a two wire implementation would normally lead to an unacceptable current consumption in view of available power supply all through the control bus. These restrictions are becoming even more severe in view of the fact that control buses and current loops will be operative with even more reduced currents—e.g., as low as 3.6 mA.